The determination of solids level in process vessels can be difficult and expensive. This is especially so in liquid-filled continuous conventional cellulosic chips digesters, such as shown in U.S. Pat. No. 3,579,421. Commercially available techniques for detecting such chips levels are not entirely successful. It is not possible to detect the level by density difference since the density of the chip mass and that of the liquors to similar, and various mechanical structures that are utilized are expensive to construct and to install, and once installed are difficult to maintain.
According to the present invention, a method and apparatus are provided that can determine the solids level in fluid-filled vessels, particularly continuous pulp digesters, that are relatively easy to construct, install, and maintain and can determine the solids level within the necessary degree of accuracy for such process vessels. The invention is based on the fact that the solids are sound absorbing while the fluid in the fluid phase is sound transmitting. The level of solids is then indicated by the difference in energy pick-up between any two consecutive receivers that are vertically spaced along the vessel wall, or the difference in energy picked up by a single receiver in response to the sequential actuation of two consecutive generators that are vertically spaced along the vessel.
According to the present invention, a solids level detecting assembly is provided comprising a fluid-filled vessel adapted to have sound absorbing solid particles flowing downwardly therein with a sound conducting fluid in a fluid phase above the level of solids, and having a generally vertical axis; a plurality of sound receivers mounted at spaced vertical positions to the exterior of the vessel; means for periodically generating sound waves in the fluid phase of the vessel above the level of solids particles; and means for evaluating the sound energy received by the receivers in response to the generated sound waves to determine the level of solids relative to two or more of the receivers. The sound generating means is preferably mounted on the exterior of the vessel substantially 180.degree. around the circumference of the vessel from the vertical line of sound receivers, and the generating means may comprise a vessel impacter or means for electrodynamically vibrating the vessel, such sound generating means acting at a fluid phase portion of the vessel. Alternatively, the sound generating means may generate sound within the fluid in the vessel fluid phase, such as by providing a diaphragm in contact with the fluid, which diaphragm is accessible from the vessel exterior, however, for ready maintenance. The receivers and sound generator can all be mounted to the exterior of the vessel with the vessel in operation, thereby making the installation simple even for retrofitting.
Preferably, a filter is associated with the receivers and the evaluating means for reducing the response of all sound received by the receivers except that sound at a particular predetermined frequency most suited for the particular vessels and solids involved. For liquor-filled continuous cellulosic chips digesters, it has been found that the optimum frequency that will be absorbed by the chips is about 3200 Hz. The receiver is tuned to view only sound at such an output, and the sound generating means can also be provided to emit sound at that particular frequency. A tuner and output display can be associated with each receiver, however, in order to save on equipment expense, it is preferred that the evaluating means include timing means for cyclically sampling each receiver output in turn with a band pass filter disposed in the output from the timing means for filtering the receiver output, a vibration envelope detector for detecting the peak of the receiver output, and a single display means for displaying the peak detected by the detector for each receiver in turn. The display means preferably complies as a multi-channel chart recorder, although a number of indicator lights or other appropriate instrumentation may be provided.
Alternatively, the detecting assembly includes a plurality of sound generators mounted at spaced vertical positions along the vessel, a single sound receiver mounted in operative association with the fluid phase of the vessel, above the level of solids, and means for evaluating the sound energy received by the receiver in response to the generated sound waves to determine a level of solids relative to two or more of the sound generators. Timing means are provided for cyclically actuating each of the sound generators in turn. The sound receiver, which preferably comprises an accelerometer, utilizes a vibration envelope detector for detecting the peak of accelerometer output in response to each sequential actuation of a generator, and for displaying the peak detected by the detector for each generator.
According to the method of the present invention, it is possible to determine the level of sound absorbing solids in a sound conducting fluid-filled vessel having a fluid phase above the level of solids. The method comprises the steps of providing a plurality of sound receivers at spaced vertical positions along the vessel; periodically generating sound waves in the fluid phase portion of the vessel above the solids level; receiving sound energy with the receivers in response to the generated sound waves; and evaluating the sound energy received to determine the level of solids relative to two or more of the sound receivers. For the receivers below the solids level, the sound energy received will be low, while for the receivers above the solids level, the sound energy received will be high, the solids level being readily determined to be between adjacent sound receivers that have low and high energy received thereby. The sound waves may be generated cyclically or intermittently, by acting on the vessel itself or by generating sound with the fluid of the fluid phase.
When a plurality of generators are utilized, and a single receiver, for the generators below the solids level, the sound energy received by the receiver will be low, while for the generators above the solids level, the sound energy received by the receiver will be high, the solids level being readily determined to be between the adjacent sound generators that result in low and high energy being received by the receiver.
It is the primary object of the present invention to provide a practical manner of detecting the level of solids in a process vessel, particularly the level of cellulosic chips in a continuous digester. This, and other objects of the invention, will become clear from an inspection of the detailed description of the invention, and from the appended claims.